An engine misfires under acceleration due to faults in the spark plugs, plug wires or coils, or a weak fuel pump. Broken rods, pistons or crank bearings, cooling difficulties, and failed or distorted valves may also cause the engine to misfire.
A weak fuel pump delivers inadequate pressure to the engine and causes it to run lean, hesitate and misfire when accelerating. Corroded or loose wiring connections, a low system voltage and a bad relay, which affect the fuel pump, usually cause the engine to stumble. Most of the causes of engine misfires result from failure to address simple problems in the injection or ignition.